1. Field of the Invention
The present invention relates to a motor control circuit for controlling a direct-current motor by use of a drive circuit based on the BTL (balanced transformer less) method.
2. Description of the Prior Art
Optical disc players include players for CDs (compact discs), CD-ROMs, DVDs (digital video discs), DVD-ROMs, SD-ROMs (super-density ROMs), LDs (laser discs), MDs (minidiscs), and similar optical discs. In optical disc players, as signals are read from or write to an optical disc, the optical disc is rotated by a spindle motor, and an optical pickup is moved by a sled motor.
Conventional optical disc players employ a drive circuit based on the BTL method as shown in FIG. 1. In FIG. 1, the direct-current motor 101, which is a motor having brushes, corresponds to a spindle motor or sled motor used in an optical disc player. Typically, an optical disc player is provided with separate drive circuits to control a spindle motor and a sled motor independently.
The drive circuit 92 shown in FIG. 1 operates as follows. The drive circuit 92 receives at its input terminal 105 a control voltage from a servo IC 91. The control voltage is fed through a resistor Rin to the inverting input terminal (-) of an operational amplifier 103. The operational amplifier 103 receives at its non-inverting input terminal (+) a reference voltage (Vref). The voltage difference between the control voltage and the reference voltage (Vref) is represented by Vin. The operational amplifier 103 has a feedback resistor RNF connected between its inverting input terminal (-) and output terminal, so that the output voltage of the operational amplifier 3 is EQU V1=-(RNF/Rin)Vin+Vref.
This voltage V1 is fed through a resistor R to the inverting input terminal (-) of another operational amplifier 102. The operational amplifier 102 receives at its non-inverting terminal (+) the reference voltage (Vref). Moreover, the operational amplifier 102 has a feedback resistor R connected to its inverting input terminal (-), so that the output voltage of the operational amplifier 102 is EQU V2=-(V1-Vref).
This voltage V2 is output via an output terminal 106. On the other hand, the voltage V1 is outputted via another output terminal 107. The motor 101 is connected between these output terminals 106 and 107. Thus, when a voltage Vout is applied to the motor 101, the motor 101 rotates. Here, Vout is expressed as ##EQU1##
Accordingly, the voltage Vout applied to the motor 101 varies according as the voltage Vin varies, and this voltage Vin varies according as the control voltage fed from the servo IC 91 varies. In some applications, the servo IC 91 is controlled by instructions given from software running on a microcontroller 90.
As shown in the broken-line square 125, the motor 101 can be regarded as equivalent to a circuit composed of armature resistance Ra and back electromotive force (voltage) Ea connected in series. Here, the back electromotive force Ea is proportional to the rotation rate N (i.e. number of revolutions per unit time) of the motor 101. Accordingly, the current that flows through the motor 101 is expressed as EQU Ia=(Vout-Ea)/Ra, (1)
and the back electromotive force Ea, which is proportional to the rotation rate N, is expressed as EQU Ea=K.multidot..PHI..multidot.N.
(Here, K is the constant of proportionality, and .PHI. is the effective magnetic flux per pole.) Hence, the rotation rate N is expressed as EQU N=Ea/(K.multidot..PHI.). (2)
Further, from expressions (1) and (2), the following expression is obtained: EQU N=(Vout-Ia.multidot.Ra)/(K.multidot..PHI.). (3)
As described above, the voltage Vout applied to the motor 101 is controlled in accordance with the control voltage from the servo IC 91. However, as expression (3) indicates, this does not mean that the rotation rate N is directly controlled in accordance with the control voltage; in reality, it is controlled as follows. Consider, for example, a transient state of the motor 101, as when the motor has just been started up. When the motor 101, after having been completely at rest, receives a voltage Vout and thus starts to rotate, the back electromotive force Ea does not appear at first. Accordingly, at this time, the current Ia is larger than predicted from expression (1), and thus the rotation rate N is lower than predicted from expression (3). Then, gradually, the rotation rate N increases, and the current Ia decreases accordingly, until the motor comes to rotate at a constant rotation rate N.
In an optical disc player, in order to improve the rate at which signals are read from or written to an optical disc, it is essential to improve the response of the motor 101.
However, in conventional optical disc players employing the drive circuit 92, only the voltage Vout to be applied across the motor 101 can be controlled. As a result, the response cannot be improved beyond limits imposed by factors such as the inertia of the motor 101. To overcome this problem, in conventional optical disc players, it is customary to force the microcontroller 90 to vary the control voltage disproportionately when the rotation rate of the motor 101 needs to be varied greatly, as when the motor 1 has just been started up. For example, it is possible to start up the motor 101 more quickly by forcing the microcontroller 90 to demand a disproportionately high control voltage to obtain an accordingly higher output voltage Vout. However, such control requires the microcontroller 90 to perform complicate operations. Moreover, raising the control voltage means increasing the gain and thus tends to result in unstable operation of the drive circuit 92 due to oscillation or other undesirable condition. Accordingly, the gain cannot be increased beyond a certain limit.
Furthermore, when the load of the motor 101 as seen from the output terminals 106 and 107 varies, this drive circuit 92 cannot cope with it. To solve this problem, in conventional optical disc players, as the microcontroller 90 reads signals from the optical disc, it checks whether those signals have been read correctly, and instructs the servo IC 91 to adjust the control voltage appropriately. However, to achieve this, the microcontroller 90, the servo IC 92, and the drive circuit 92 all need to be involved in the control of the motor 101. Such control requires complicate operations, and thus the response cannot be improved beyond a certain limit.
Ideally, the motor 101 should be controlled by directly controlling its rotation rate N. In addition, the motor 101 needs to reach the desired rotation rate N as quickly as possible. However, in conventional optical disc players employing the drive circuit 92, the rotation rate N cannot be controlled directly. As a result, the response cannot be improved beyond a certain limit. Moreover, the drive circuit 92 cannot cope with variation in the load. As a result, the microcontroller 90 and the servo IC 91 need to perform complicate operations, and thus, also for this reason, the response cannot be improved beyond a limit.